This invention relates to the protection of load cells from overloads and shock loads, and more particularly to a novel apparatus which allows substitution of load cells having substantially different stiffness without the need to redesign the over load protection for the load cell used.
The use of load cells in scales has become quite common. Load cells offer many advantages in comparison to spring and balance scales; particularly, they provide an electrical output which is readily adaptable to the electronics of computing scales (i.e., scales having an output which is a function of the measured weight and other variables) such as postal scales. However, load cells have a significant disadvantage in that they are easily damaged by overloads and shock loads. Thus, it is necessary to provide overload and shock load protection for load cells in the form of downstops, which limit the deflection of the load cell under load. Upstops are also frequently provided to limit upwards deflection and protect against lifting loads, as when a scale is lifted by the pan. Typically load cells are also protected against torsion loads (e.g., from twisting forces generated by objects placed off center on the scale pan) by downstops positioned under the corners of the scale pan. Methods of torsion load protection are well understood by those skilled in the art and may be used with the apparatus of the subject invention, but need not be discussed further here for an understanding of the subject invention.
Since load cells, even of the same model, will tend to vary slightly in stiffness it has, in general, been necessary to provide means for adjusting the overload protection stops provided in a scale. (for purposes of this discussion "stiffness" may be considered as the load per unit deflection). Further, since full scale deflection, even for the least stiff load cells, is on the order of thousandths of an inch such adjustments were delicate and particularly difficult to do in the field. Typically the downstop would be adjusted by adjusting a bolt threaded into the base beneath the load cell. The bolt would then be secured with a lock nut to act as a downstop. This approach, however, has several problems. One problem is that securing the lock nut often disturbed the adjustment. Another problem is that in many designs the adjustment could only be made with access to both sides of the scale base. Another is that the difference in stiffness of load cells is so great that installation of a new load cell without first backing-off the downstop from its previous adjustment would often result in an upward deflection great enough to damage the new load cell. Also, such downstops provided no protection against lifting forces so that separate upstops, requiring a separate adjustment, where needed to provide protection against lifting forces.
Another problem with previous means of protecting load cells from overload and shock load is that if a scale manufacturer wanted to substitute a different model of load cell into his scale the change in full scale deflection might be so great, possibly by a factor of four or more, that the stop adjustment might lack sufficient range to accommodate the new load cell. Further, if a relatively stiff load cell, with a small full scale deflection, were substituted the required adjustment of the stops would become still more delicate and difficult to do.
Thus, it is an object of the subject invention to provide an apparatus for protecting a load cell from overload and shock load which may be simply adjusted.
It is another object of the subject invention to provide such an apparatus which is adaptable to function with load cells having different degrees of stiffness.
It is still another object of the subject invention to provide a means whereby the apparent stiffness of a load cell may be adjusted to a predetermined level without affecting its measurement characteristics.